1. Field of the Invention
The present invention relates to a method of producing of a cellulose ester film which is used for a deflector protect film and an optical compensation film.
2. Description Related to the Prior Art
A deflector protect film and an optical compensation film are produced of a cellulose ester film, such as cellulose triacetate film (hereinafter TAC film) and the like. The TAC film is produced in a method for producing a film from a solution. In the method additives are solved in a solvent to produce a high molecular solution (dope solution), and thereafter the dope solution is doped on a supporter with a die having a slit, dried for a predetermined time, peeled from the supporter, and further dried again to produce the TAC film.
When the dope solution is dried for a long time thereby, the solvent in the dope solution is evaporated to form a skinning of high molecular material on both edges of the slit of the die. The skinning prevents the doping of the dope solution, and causes the thickness in both edges of the dope solution on the supporter to become uneven. Further sometimes the skinning falls down from the both edges of the slit of the die. In this case, the skinning adheres on a feed roller to damage the TAC film dried from the dope solution on the supporter. In order to prevent the generation of the skinning, U.S. Pat. No. 3,112,528, Japanese Patent Laid-Open Publications No. 2-208650 and 5-86212 describe methods in which the solvent is flown to solve solutes in the dope solution or a mixture of the evaporated solvent and air are blown.
However, in the above methods, the amount of the mixture and the solvent is hardly regulated. Accordingly vibration of edges of a dope ribbon between the die and the supporter causes to generate the skinning.
Further, the deflector protect film is applied to one of both side of a deflector. The deflector is formed of a polyvinylalcohol film (hereinafter PVA film). The PVA film is extended on a shaft for dieing in iodine or to color dyne, and otherwise after dying the PVA film is extended, and bridges with boron compound to produce the deflector.
Several types of the cellulose ester films are proposed. For example, Japanese Patent Laid-Open Publication No. 6-134933 proposes a cellulose triacetate laminate film having a cellulose triacetate film produced from cotton and pulp for easy peeling and improving the heat resistance. Further, Japanese Patent Laid-Open Publication No. 2000-19776 proposes a cellulose ester film whose averaged substitution degree is 2.88-3.00 to always prevent the generation of optical creases.